Multipole magnets and particularly quadrupole magnets have been found useful for a variety of applications including, for example, focusing charged particle beams. Conventionally, electromagnets have been used for such multipole configurations because of the limitations of the field strength of permanent multipole magnets and because the field strength of electric magnets could be easily varied by controlling the current density whereas the field strength of permanent magnets is fixed.
Rare earth-cobalt (REC) materials have renewed interest in permanent magnet multipoles. Most of the work has been done with respect to quadrupole magnets. For the past several years there has been considerable effort in developing permanent magnet quadrupoles for replacing electromagnets, particularly in applications such as the drift tubes in proton linacs. See, for instance, Murin et al, Inst. Exp. Tech., 19 (2) (1976); and Saito et al., Proc. Third Int. Workshop REC Perm. Mag. and Appl., (1978). Such designs were primarily based on replacing the coils in an electromagnet quadrupole with four suitably oriented pieces of samarium cobalt. In one such prototype permanent magnet quadrupole having an aperture radius of 1.3 cm., a pole tip field of about 3.0 Kilogauss was obtained. A method and design for reducing flux leakage in permanent magnets was described in U.S. Pat. No. 3,768,054. These previous types of permanent magnet quadrupoles apparently have limitations to about 6 Kilogauss pole tip fields using the best commercially available REC materials.
Recently a new design for permanent magnet quadrupoles was described. See Halbach, "Strong Rare Earth Cobalt Quadrupoles", LEEE Trans, Nucl. Sci., (June 1979), Holsinger et al., "A New Generation of Samarium - Cobalt Quadrupole Magnets for Particle Beam Focusing Applications", Proc. Fourth Int. Workshop REC Perm. Mag. and Appl., (1979) and Halbach, "Design of Permanent Multi pole Magnets With Oriented Rare Earth Cobalt Material", Nucl. Inst. Meth., 169, pp. 1-10 (1980), which are hereby incorporated by reference. The new design for REC quadrupoles allows construction of compact quadrupoles with magnet aperture fields of at least 1.2 tesla (T) with presently available materials.
The development of REC materials was begun by Strnat around 1966. Currently available materials are available as a sintered block of small, oriented, highly anisotropic crystals (composed of about one part rare earth metal and five parts cobalt) strongly magnetized in the preferred crystaline direction, conventionally called the "easy axis".
The new design comprises a ring quadrupole having a continuously varying easy axis orientation to keep all of the flux within the ring of material except for the aperture field. This theoretical ring quadrupole is approximated with a segmented ring quadrupole. For convenience of construction a 16 piece quadrupole is described by Holsinger et al. supra. This construction requires REC materials having only four different easy axis orientations.
To realize the advantages of permanent magnet quadrupoles in large aperture beam line magnets, two significant problems need to be solved: (1) the quadrupole focusing strength must be adjustable in most applications, and (2) the cost of the REC pieces must be controlled so that the total cost of the quadrupole assembly will be comparable to that of an electromagnet including the power supply.